This invention is a further development in the field of cryogenic pumping, and in particular provides a controllably variable pumping speed for a selected gas from a chamber to be evacuated while maintaining a substantially constant pumping speed for a different gas having a higher condensation temperature than the gas whose pumping rate is to be varied.
The invention finds particular application in sputtering systems in which it is advantageous to be able to vary the speed at which a gas such as argon can be removed from the sputtering chamber while maintaining a constant pumping speed for water vapor.
In the prior art, it was known to use a throttling valve disposed in a coupling structure between the chamber to be evacuated and a pump that removes the gas whose pumping speed is to be varied. By controlling the opening and closing of the throttling valve, the speed at which the selected gas (e.g., argon) was removed from the chamber could be controlled. However, this throttling technique, while providing the desired pumping speed for the particular gas of concern, also affected the pumping speed of other gases in the chamber (e.g., water vapor).
It was likewise known in the prior art to dispose a liquid nitrogen cooled Chevron baffle or other type of cold trap in the flow path from the chamber being evacuated to the throttling valve. However, the liquid nitrogen reservoir was disposed upstream of the throttling valve in the prior art.